News

In my January column (www.amos.org.au/news/id/105) I pointed out that the 2010/11 La Niña event looked like being the strongest or second strongest event on record, based on data stretching back to the late 19th century. It was premature, at that time, to decide conclusively just where the event would rank, since it was still continuing. With summer now finished, and some data suggesting that the event may be on the decline, it is worth revisiting this topic, and to examine whether the La Niña was the only phenomenon influencing the disastrous rains and floods we have seen through spring and winter 2010/11. In particular, is there a global warming signal evident in the data?

The graph below shows the time series, since 1900, of spring and summer total rainfall averaged across eastern Australia (average rainfall over Queensland, New South Wales, Victoria and Tasmania), from the Bureau of Meteorology’s excellent web pages on climate change (the blue line). September-February 2010/11 was the second wettest spring and summer we have seen, at least since 1900 (the wettest was 1973/74). The Southern Oscillation Index (SOI, the normalised Tahiti-Darwin pressure difference, also taken from the Bureau’s web pages) is also shown in the graph (red line) and 2010/11 was the strongest maximum observed since at least 1900. So we can confidently conclude that 2010/11 La Niña event was indeed the strongest on record. Given the well-known relationship between the SOI and heavy rains in eastern Australia (eg., McBride and Nicholls, 1983) we can conclude that the fundamental cause of the heavy rains this past six months was indeed this record La Niña event. Other heavy rain years (1917/18, 1950/51, 1973/74, 1975/76) were also the result of strong La Niña events. The relationship between rainfall and the SOI is very strong, with a correlation coefficient of 0.66. So, the heavy rains were not caused by global warming, but by a record la Niña event – a natural fluctuation of the climate system.

But perhaps 2010/11 was a record La Niña because of global warming? There has not been any trend in the SOI over the past 111 years, despite the warming of global mean temperature of about 0.75°C over that period. Nor do climate models consistently predict increased strength of La Niña events from enhanced atmospheric content of greenhouse gases (eg., Vecchi and Wittenberg, 2010). So there is no reason, at this moment, for us to suspect that global warming is increasing the frequency or intensity of La Niña events.

But was the impact of the 2010/11 La Niña on Australian rainfall stronger because of the record warm sea surface temperatures around northern Australia in 2010? These waters have increased substantially over the last century and are now about a degree warmer than early in the 20th century. If these warmer waters were enhancing the impact of La Niña on Australian rainfall we might expect to be seeing heavier rains in recent decades, relative to the rains that accompanied earlier strong La Niña events. There is some evidence of this (eg., Nicholls et al 1996), and there has been a weak tendency towards increased rainfall since 1900, independent of the influence from the El Niño – Southern Oscillation. Perhaps this trend towards increased rainfall might be related to the warmer sea surface temperatures – but much more work is needed to test this. The effect, if there is one, does not look very strong.

I repeated this analysis just using Queensland rainfall, and reached the same conclusion. The record La Niña event was the fundamental cause of the heavy rains and floods, ie it was a natural fluctuation of the climate system. There may be a global warming signal enhancing this natural variability, but if so then this effect has been quite subtle, at least thus far. The dominant influence on Australian rainfall variations is the El Niño – Southern Oscillation, and it is this influence that makes Australia a land of “droughts and flooding rains” (Nicholls, 1988).